The fracture of fan blades during engine operation, sometimes termed “fan blade-off”, is a significant safety concern. Such events may be caused by the collision of a foreign object into the fan. The very high relative velocity at which the fan blades and a foreign object collide can be sufficient to result in fracture of an impacted blade. In the event of such a fracture, fragments may be expelled at high velocity under centrifugal force potentially colliding with other components of the fan.
A consequence of loss of all or part of a fan blade is that the fan becomes unbalanced and its shaft tends to orbit an axis about which it is designed to rotate. This can result in a significantly increased radial load on downstream components such as a rotor of the compressor of the gas turbine engine. Excess radial loads can result in a mainline shaft failure, decoupling the turbine from the fan. In such circumstances the decoupled turbine has a tendency to overspeed, potentially leading to failure of a turbine disc.
In some engine designs, in the event of a mainline shaft failure rearwards movement of the rear section of the shaft can be limited due to a clash with the front bearing housing. In such designs, the bearing race can be connected to the mainline shaft assembly via a stub shaft. A fan catcher assembly is typically bolted to the stub shaft on an upstream facing side and to the bearing sleeve assembly on a downstream facing side. In the event of a fan-shaft failure or fan blade off event causing damage or failure to the fan-shaft, the fan-catcher assembly is intended to arrest any forward movement (i.e. in a downstream direction) of the fan-shaft by impacting on a downstream face of the bearing structure. The resulting reaction load is taken through the bearing to the upstream face of the bearing housing structure. As the axial load is applied to the fan catcher ring during a fan shaft failure, the clamp load on the fan catcher ring is reduced and this may allow slippage on the axial face between the stub shaft and the catcher ring. This prevents bending of the fan catcher arm which helps to restrict the axial deflection of the ring.
FIGS. 1 and 2 illustrate a known configuration of the fan end of a gas turbine engine. FIG. 2 illustrates a failure consequent on a fan blade-off event in the arrangement of FIG. 1.
In the arrangement of FIGS. 1 and 2 a fan 1 is mounted on a fan shaft 2 which constitutes part of a mainline shaft assembly centered for rotation on an engine axis X-X. When in operation, the fan rotates about the engine axis X-X. A stub-shaft 3 is arranged radially outwardly and concentrically with the fan shaft 2. A fan catcher arm 4 extends from the stub-shaft 3 through a bearing sleeve 5 and joins a fan catcher ring 6 arranged downstream of the bearing housing 5 and having an upstream facing face arranged to abut against a downstream facing face of the bearing housing 5 in the event of a fan blade-off. Immediately downstream of the fan catcher ring 6 is formed a shaft end 7 which is threaded to receive a nut (not shown) that holds the shaft onto the fan catcher ring 6. As can be seen the stub shaft 3 engages with the fan shaft 2 at a position 8 at an upstream end of the fan shaft 2. In contrast, the fan catcher arm 4 is radially distanced from the fan shaft 2 by a distance d which is typically of the order of 5-10 mm. Another shaft end 9 extends axially downstream from the bearing sleeve 5 and is threaded for a nut (not shown) to be attached that holds onto the bearing sleeve 5. The mainline shaft assembly constitutes the fan shaft 2 (which extends to become the compressor shaft) and a turbine shaft.
In the event of a fan blade-off event, unbalance in the fan results in the fan shaft 2 and surrounding fan catcher arm 4 tending to orbit the engine axis X-X rather than rotate on it. Thus when a critical radial load R is reached, the fan catcher arm 4 deflects and is thrown against the bearing sleeve 5 resulting in a fracture 10 of the fan catcher arm 4. Consequential damage can result to engine components downstream (not shown) with an associated risk of engine failure.